Lignin, one of the main components of wood, has been used in many applications in the form of lignin derivatives by providing various modifications so far. For example, lignin sulfonic acid has widely been used as a cement dispersant. Also, for example, Japanese Patent Application Laid-Open Publication No. 1-145358 discloses making lignin sulfonic acid in the form of acrylic or vinyl monomer-modified derivatives for use of lignin sulfonic acid as a cement dispersant. On the other hand, U.S. Patent Application Publication No. 2002/0065400 discloses a resin composition mainly for binders, which is obtained by grafting kraft lignin with acrylic acid and crosslinking the obtained polymer with aldehydes. Further, U.S. Pat. No. 4,891,415 discloses a resin composition particularly suitable as dispersants and scale preventing agents, which is obtained by grafting lignin, for example, lignin sulfonic acid, with vinyl monomers.
However, in the case where the above-described conventional lignin derivatives, e.g., lignin sulfonic acid, are used as dispersants, there has been a problem that they do not have so sufficient cement dispersing performance. Therefore, it has been required to effectively improve the dispersing performance, which is intrinsic to the lignin skeleton, by chemical modification and efficiently utilize the resulting lignin derivatives as dispersants having high dispersing performance. Further, it has also been required to develop a novel lignin derivative from kraft lignin which has been used only as fuel in pulp plants.